Windshields and rear windows of vehicles such as automobiles often include electrical devices located within or on the glass. Typically, the electrical devices are antennas or defrosters. In order to provide an electrical connection to such an electrical device, a small area of metallic coating is applied to the glass to make the metalized surface which is electrically connected to the electrical device. An electrical connector is then soldered onto the metalized surface. The electrical (i.e., power) connector is commonly soldered to the metalized surface of glass with a solder that contains lead (Pb). Due to environmental concerns and/or regulatory mandates in various countries, most industries are currently using or planning to use non-lead solders in soldering applications. A common non-lead solder employed in some industries contains a high tin (Sn) content, such as more than 80% tin. Non-lead solders used on automotive glass as described herein are disclosed in U.S. Pat. No. 6,253,988 issued to John Pereira on Jul. 3, 2001 (hereinafter “Pereira”). Among several non-lead solders, Pereira discloses a solder composition with a weight percentage of 64.35%-65.65% indium (In), 29.7%-30.3% tin (Sn), 4.05%-4.95% silver (Ag), 0.25%-0.75% copper (Cu) (hereinafter the “65 Indium Solder”).
There are difficulties encountered when soldering devices to automotive glass that are not present in other applications. Automotive glass tends to be brittle, and the common high tin, non-lead solders that are suitable for use in other applications can typically cause cracking of the automotive glass. Although materials such as ceramics and silicon might appear to be similar in some respects to automotive glass, some solders that are suitable for soldering to ceramic or silicon devices are not suitable for soldering to automotive glass. Soldering two materials with a substantial difference in coefficient of thermal expansion (CTE) between them, such as glass and copper in this case, imposes stress on the solder, either during cooling of the solder joint, or during subsequent temperature excursions. The solder composition needs to have a melting point (liquidus) that is low enough to not cause cracking of the automotive glass during the soldering process, because a higher melting point and correspondingly higher processing temperature augments the adverse effects of CTE mismatch, imposing higher stress during cooling. The melting point of the solder composition, however, needs to be high enough not to melt during the normal use of a car, for example, when the car is in the sun with the windows closed or under other extreme harsh environmental conditions. Solders that contain indium, however, normally have much lower melting points than other solders. The 65 Indium Solder, for example, has a solidus temperature of 109° C., compared to 160° C. of the lead solder, and a liquidus temperature of 127° C., compared to 224° C. of the lead solder. Some vehicle manufacturers desire that glass products should be capable of surviving elevated temperatures, for example 110° C. for one original equipment manufacturer (OEM) and 120° C. for another, without any deterioration in performance.
Therefore, there is a need for a non-lead solder composition suitable for use on glass that can withstand higher elevated temperatures than compositions currently available, while delivering all other desired properties for this application sector.